Influenza A viruses (IAV) are significant human pathogens causing yearly epidemics and occasional pandemics. Past pandemics have resulted in significant morbidity and mortality. The 1918 influenza pandemic was thought to have resulted in the death of at least 675,000 people in the U.S., and 40 million people worldwide. Pandemics in 1957 and 1968, while less severe, were also of major public health importance. A novel influenza A virus of swine origin became pandemic in 2009, causing the first pandemic in 41 years. In addition, annual epidemic influenza causes are also very significant resulting in up to 49,000 deaths in the US annually. A variety of experimental pathogenesis studies to achieve these goals were performed this year. Obesity has been identified as an independent risk factor for severe or fatal infection with 2009 pandemic H1N1 influenza, but was not previously recognized for previous pandemic or seasonal influenza infections. Our aim was to evaluate the role of obesity as an independent risk factor for severity of infection with 2009 pH1N1, seasonal H1N1, or a pathogenic H1N1 influenza virus. Diet-induced obese (DIO) and their non-obese, age-matched control counterparts were inoculated with a 2009 pH1N1 influenza virus, a seasonal H1N1 virus, or a classical swine H1N1 virus. DIO mice had higher mortality (80%) than control mice (0%) and lost more weight during pH1N1 infection. No effect of obesity on morbidity and mortality was observed during seasonal or or swine influenza infections. Influenza antigen distribution in the alveolar regions of the lungs was more pronounced in DIO than control mice during pH1N1 infection and localized interferon-&#946; and proinflammatory cytokine protein responses in the lungs were significantly lower in DIO than control mice. Conversely, serum cytokine concentrations were elevated in DIO, but not control mice following infection with CA/09. The effect of obesity on differential immune responses was abrogated during seasonal or swine H1N1 infection. Together, these data support epidemiologic reports of obesity as a risk factor for severe 2009 pandemic H1N1 influenza infection. The 1918 influenza pandemic caused 50 million deaths worldwide. Understanding the origin, virulence, and pathogenic properties of the 1918 virus, is crucial for current public health preparedness and future pandemic planning. The origin of the 1918 pandemic virus has not been resolved, but its coding sequences are very like those of avian influenza virus. The proteins encoded by the 1918 virus differ from typical low-pathogenicity avian influenza viruses at only a small number of amino acids in each open reading frame. To study this, a series of chimeric 1918 influenza viruses were created in which each of the eight 1918 pandemic virus gene segments was replaced individually with the corresponding gene segment of a prototypical low-pathogenicity avian influenza (LPAI) H1N1 virus in order to investigate functional compatibility of the 1918 virus genome with gene segments from an LPAI virus and to identify gene segments and mutations important for mammalian adaptation. This set of eight 7:1 chimeric viruses was compared to the parental 1918 and LPAI H1N1 viruses in intranasally infected mice. Seven of the 1918 LPAI 7:1 chimeric viruses replicated and caused disease equivalent to the fully reconstructed 1918 virus. Only the chimeric 1918 virus containing the avian influenza PB2 gene segment was attenuated in mice. This attenuation could be corrected by the single E627K amino acid change, further confirming the importance of this change in mammalian adaptation and mouse pathogenicity. While the mechanisms of influenza virus host switch, and particularly mammalian host adaptation are still only partly understood, these data suggest that the 1918 virus, whatever its origin, is very similar to avian influenza virus. Highly pathogenic H5N1 influenza shares the same neuraminidase (NA) subtype with the 2009 pandemic H1N1, and cross-reactive NA immunity might protect against or mitigate lethal H5N1 infection. To evaluate this, mice were either infected with a sublethal dose of 2009 pandemic H1N1 or were vaccinated and boosted with virus-like particles (VLP) consisting of the NA and matrix proteins, standardized by NA activity and administered intranasally, and were then challenged with a lethal dose of HPAI H5N1 virus. Mice previously infected with 2009 H1N1 survived H5N1 challenge with no detectable virus or respiratory tract pathology on day 4. Mice immunized with H5N1 or H1N1 NA VLPs were also fully protected from death, with a 100-fold and 10-fold reduction in infectious virus, respectively, and reduced pathology in the lungs. Human influenza vaccines that elicit not only HA, but also NA immunity may provide enhanced protection against the emergence of seasonal and pandemic viruses. Influenza A virus infection leads to variable and imperfectly understood pathogenicity. We reported that segment 3 of the virus contains a second open reading frame (X-ORF), accessed via ribosomal frameshifting. The frameshift product, termed PA-X, comprises the endonuclease domain of the viral PA protein with a C-terminal domain encoded by the X-ORF and functions to repress cellular gene expression. PA-X also modulates IAV virulence in a mouse infection model, acting to decrease pathogenicity. Loss of PA-X expression leads to changes in the kinetics of the global host response, which notably includes increases in inflammatory, apoptotic, and T lymphocyte-signaling pathways. Thus, we have identified a previously unknown IAV protein that modulates the host response to infection, a finding with important implications for understanding IAV pathogenesis.